1. Technical Field
This invention relates to electric arc spraying of metals and, more particularly, to a plasma-arc transferred to a single wire tip that is fed continuously into the plasma-arc.
2. Discussion of the Prior Art
As disclosed in earlier U.S. patents by the co-inventors herein, plasma transferred wire arc is a thermal spray process which melts a continuously advancing feedstock material (usually in the form of a metal wire or rod) by using a constricted plasma-arc to melt only the tip of the wire or rod (connected as an anodic electrode); the melted particles are then propelled to a target. The plasma is a high velocity jet of ionized gas which is desirably constricted and focused about a linear axis by passing it through a nozzle orifice downstream of a cathode electrode; the high current arc, which is struck between the cathodic electrode and the anodic nozzle, is transferred to the wire tip maintained also as an anode. The arc provides the necessary thermal energy to continuously melt the wire tip, and the plasma provides the dynamics to atomize the molten wire tip into highly divided particles and accelerate the melted particles as a stream generally along the axis of the plasma. Acceleration of the particles is assisted by use of highly compressed secondary gas, directed as converging gas streams about the plasma-arc axis, which streams converge at a location immediately downstream of where the wire tip intersects the plasma-arc, but avoid direct impingement with the wire tip to prevent excessive cooling of the plasma-arc.
Existing torches and associated apparatus of the prior art, used to generate the plasma transferred wire arc, lack robustness and are sensitive to instabilities in process parameters resulting in spitting of melted metal rather than spraying of fine particles. Process instabilities occur when one or more of the following are outside of controlled or designed ranges: secondary air flow or pressure, plasma gas pressure, wire feed rate, wire current, and torch movement rate. The occurrence of such instabilities are not fully predictable and can occur early or late in the operational life of the torch.
Spitting results from the accumulation of melted particles which tend to agglomerate and form globules or droplets that move back up along the wire under the influence of fluid dynamics of the plasma jet and secondary gases. Such globules or droplets can contaminate the wire tip and/or release the globules for projection that produces a nonuniform deposit. Process instabilities, that allow particles to agglomerate, may have their origin in a change of electrode shape over time due to wear, buildup of contaminants, or due to irregularities such as the rate of wire feed by the automatic feeding mechanism or changes in the level of current passing through the wire. Such process instabilities correlate with increasing periods of continuous use and higher rates of deposition.
It is an object of this invention to improve the plasma transferred wire arc process so that it may be operated more robustly to obtain high quality deposits and/or faster deposition rates without any reduction in quality of the deposit.
The invention, in a first aspect, is a method of thermally depositing metal at increased rates onto a target surface, comprising: establishing a high velocity plasma transferred wire arc between a cathode and the free-end of a consumable wire electrode, the energy of such plasma and arc being sufficient to not only melt and atomize the free-end of the wire into fine metal particles, but also project the particles as a column onto such surface at an enhanced deposition rate; surrounding the plasma and arc with high velocity and high flow gas streams that converge beyond the intersection of the wire free-end with the plasma-arc, but avoid direct impingement on the wire free-end; and impinging a low velocity gas flow on the advancing wire to counteract any destabilizing fluid dynamic forces attempting to move the melted metal particles back along the wire away from the wire free-end.
The invention, also, is an improved apparatus for coating a target surface with a dense metallic coating using a plasma transferred wire arc metal spraying process, the apparatus including a cathode, a nozzle generally surrounding a free and of the cathode in spaced relation and having a restricted orifice opposite the cathode to form a plasma, a wire feed mechanism that directs a free end of a wire feedstock into the plasma, a source of electrical energy for striking an arc between the cathode and nozzle for transfer to the free end of the wire, the apparatus further comprising; a plurality of high velocity and high flow gas ports in the nozzle arranged annularly about the orifice to direct secondary gas streams that surround the plasma-arc and converge with respect to the plasma-arc axis at a location beyond the wire free-end but which do not impinge directly on the wire free-end; and means providing at least one low velocity gas flow that directly impinges near the wire free-end to counteract any dynamic vector forces urging the melted particles back along the wire.